Copy/paste attempt to make a "get user emails" API

[turt2live]

Disclaimer: I don't know rust, and haven't tested this.

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[sandhose]

Thanks for this, I did a proper implementation in #4001

Main difference in the implementation is that it's not under the /users API, but rather in its own path, as a paginated list, with an (optional) filter[user]=ID. This will let us introduce other filters down the line more easily (like email provider domain?) and list all the emails on the whole instance